Coating compositions for metal substrates that are applied prior to painting, and that contain a hexavalent-chromium-providing substance, may further contain a finely divided metal. For example, in U.S. Pat. No. 3,671,331 there are disclosed pulverulent metal containing chromium bonding compositions that contain, in addition to the chromium substance, a reducing agent for such substance and a particulate metal such as pulverulent zinc. Also U.S. Pat. No. 3,687,739 discloses the preparation of a treated metal surface wherein such treatment includes application of a composition containing, among other constituents but as critical ingredients, chromic acid and a particulate metal.
In composite coating techniques, it has been known to dry an undercoating, which needs curing, and then apply a topcoating. A single curing step then follows to cure both the undercoating and the topcoating as has been disclosed, for example, in British Pat. No. 845,259. As has been taught in U.S. Pat. No. 4,020,220, applied coatings from compositions that contain a hexavalent-chromium-providing substance which is susceptible to valency reduction to a lower valence state during curing, may also be topcoated and then subsequently be fully cured.
After the precoat application, then the topcoat application, and the final curing, the resulting coated article must usually exhibit a wide variety of desirable characteristics. Exemplary of these are coating adhesion during metal forming operations, plus retention of weldability where the coated substrate would otherwise be weldable. Resistance to attack, and particularly as exhibited against corrosive attack of the metal substrate, is critical. Resistance to other agents, such as acids and solvents, is also often needed.